Well, it's kind of a long thing to explain... my relationship with NMM and Metallic approaches... Basically, I don't see a big difference in using NMM or metallic colours. The techniques used are the same, with the difference that when using NMM you are only concentrating in using matte paints. In my opinion, that is limiting yourself for no reason. I prefer to use as many types of contrast as I can, and if I only use metallic paints, I won't be using the contrast between shiny and matte, that is why my approach to metals normally contains both metallic paints and matte paints. I hope this helps! :)